It is well known in the surface acoustic wave art that resonators pose a particular problem in their construction. Resonators require a constant velocity throughout their structure and typically include first and second reflective gratings with first and second spaced transducers inserted between the gratings. If the transducers are very close to each other, electromagnetic cross-talk occurs because the transducers are very capacitive. Thus, they must be separated or isolated from each other electrically. When they are separated, an unmetallized region occurs between the two transducers. That region does not have the same velocity as the metallized regions, those having the transducer electrodes or the reflective gratings thereon. This means that the unmetallized region or cavity is no longer resonant at the same frequency as the gratings or the transducer. Therefore, the cavity must be changed in length to perturb the frequency. This is a very complicated process but must be accomplished since the cavity, without electrodes, causes an insertion loss and distortion of the phase response of the resonator.
Thus, a center grating must be added to cause a constant velocity through the region separating the two transducers. Further, it is important that the center gratings do not cause reflections. If .lambda./4 electrodes are used as the coupling grating, the velocity of the acoustic waves through the cavity is constant, but reflections occur from the center grating electrodes. Split-finger electrodes, well known in the art, could be used as a center grating and no reflections would occur, but then the velocity through the region would be different because there are a different number of edges per given length. Thus, with .lambda./4 electrodes, there are four edges for the two electrodes and a 50/50 metallization or 50% of the region is metallized and 50% is free space. If split-finger electrodes are used, there is still a 50/50 metallization to free space ratio, but there are now eight reflector edges instead of two. The velocity of the acoustic wave is affected by energy stored at the electrode edges and the split-finger electrode has twice as many edges. Thus, the velocity through the split-finger electrodes is different than the velocity through a structure having .lambda./4 electrodes.
Further, it is also known in such resonators that the resonators are most generally constructed with uniformly distributed reflectors. For example, all electrodes may be .lambda./4 in width and separated by .lambda./4 free space regions. The reflection characteristic of a uniform reflector has relatively strong side lobes and a large insertion loss. In order to reduce these disadvantages, withdrawal weighted reflector gratings are used. Withdrawal weighting is the selective omission, or withdrawing, of reflective elements or electrodes. Proper withdrawal weighting causes reduced reflection side lobes. However, as soon as some of the electrode fingers or elements are removed, the velocity is changed through the grating, thus creating distortion and insertion loss. In order to compensate for these disadvantages in the prior art, the remaining electrodes have to be shifted or moved to a different position on the substrate to compensate for the missing electrodes. But in order to determine where the electrodes must be positioned, one has to know the acoustic velocity on the free area compared to what it is on the metallized surface and the calculations become very complicated and precise placement of the electrodes is almost impossible. Further, the electrodes are no longer on equally-spaced grids because they vary in position nonlinearly and thus it is impossible to make such a mask with an E-beam. E-beam systems are well known in the art and basically include an electron beam to write the desired pattern on a photoresist. The E-beam operates digitally on a grid system. Further, with each different type of metal that is used or with a change in metal thickness, new calculations have to be performed because of the different velocities relating to the different metals or different thicknesses and thus a separate mask must be designed for each different type of metal or metal thickness used.
The present invention overcomes the disadvantages of the prior art by disclosing a method and apparatus for forming an electrode structure for use in the transducers and reflective gratings in a surface acoustic wave device and which has constant velocity and either no reflectivity or reflectivity that can be set to any desired value during construction of the surface acoustic wave device. In this application, the term "variable reflectivity" will be used to denote such desired value of reflectivity.
To construct such a surface acoustic wave structure having no reflections and constant velocity, multiple electrodes are placed on a piezo-electric substrate with only four spaced electrodes for each 2.lambda. distance of structure. The first two electrodes have a center-to-center spacing of 3.lambda./4 to cause wave cancellation between the first two electrodes and the second two electrodes have a center-to-center spacing of .lambda./4 to cause wave cancellation between the second two electrodes, thereby creating an electrode structure with constant velocity and no reflectivity. The structure has constant velocity because, over the entire electrode structure, only two electrodes per .lambda. are used. Thus, it is acoustically equivalent to a uniform .lambda./4 structure. It has a metal-to-free-space ratio of 50/50 and it has the same number of electrode edges as a structure having two electrodes per wavelength. Thus, the velocity through the structure is constant. It has no reflections because the first two electrodes of each 2.lambda. distance of structure are 3.lambda./8 in width and are separated by 3.lambda./8 space. Thus, the round trip distance from the center of one electrode to the other is 12.lambda./8 or 6.lambda./4 or one and one-half wavelengths. Since this is an odd multiple, the reflected wave is out of phase with the transmitted wave and cancels it. In like manner, the center-to-center spacing distance of the second two electrodes in each 2.lambda. distance of structure is .lambda./4 or one-fourth wavelength. Thus, the round trip distance is .lambda./2 or one-half wavelength and causes the reflected wave to be out of phase with the transmitted wave and the two waves cancel. Therefore, this structure has the equivalent velocity performance of a two electrode per wavelength structure, but it has no reflectivity as does the two electrode per wavelength structure. The construction of this structure can be used in either gratings or transducers to cause nonreflective gratings or nonreflective transducers but both of which have constant velocity throughout the structure. The two .lambda./4 spaces are not critical and the two .lambda./8 electrodes can be moved in unison somewhat to the right or left without changing the results.
Where it is desirable to have a transducer or grating with variable reflectivity, such as the case with external gratings where it is desired to have a tapered reflectivity to reduce side lobes and insertion loss, the structure is arranged to have two electrodes per wavelength as is the case in a structure having quarter wavelength electrodes except in this case the first electrode is 3.lambda./8 in width and the second electrode is .lambda./8 in width with separations of .lambda./4 between electrodes. The width of the two electrodes can be varied simultaneously during construction to vary reflectivity. Thus, the first electrode width may be varied from 3/8.lambda. to .lambda./4 and the second electrode width may be inversely varied from .lambda./8 to .lambda./4. It is well known that for a given distance of electrode structure, the reflectivity, utilizing quarter wavelength electrodes and spaces, approximates a sine wave. Thus, with quarter wavelength electrodes, the sine of 9.degree. equals one and the reflectivity is a maximum. However, on either side of .lambda./4, the reflectivity decreases. Thus, by making the first electrode have a width of 3/8.lambda., the reflectivity is reduced to 0.707 (.sqroot.2/2, where the angle equals 135.degree. (45.degree.)). In like manner, with the second electrode having a width of .lambda./8 (45.degree.), the reflectivity is reduced again to 0.707 (.sqroot.2/2). Thus, if the first electrode is varied in width between 1/8.lambda. and .lambda./4 while at the same time the second electrode is inversely varied in width between .lambda./8 to .lambda./4, the entire reflectivity of a given length of electrodes is reduced anywhere between a maximum and 0.707 of the maximum. Thus, an electrode structure having the desired reflectivity can be constructed.
By combining the nonreflective structure and the variable reflective structure, greater control over the amount of reflectivity of a transducer or grating structure can be controlled to obtain any desired tapering of the reflectivity.